The present invention relates to the structure of an automobile body floor panel and an automobile with this body floor panel, and in particular relates to a body floor panel that excites vibration in a specific mode with a low acoustic radiation efficiency with respect to the input of vibrations in a predetermined frequency band that results in road noise.
Road noise, that is, the noise within an automobile interior that is caused by tire cavity resonance and suspension vibration or the like while the automobile is in motion, is a problem. In general, road noise due to tire cavity resonance peaks in a specific frequency band from 200 to 300 Hz, and road noise due to suspension vibration peaks near 160 Hz. Accordingly, a variety of anti-vibration and anti-noise measures have conventionally been employed at various parts of the automobile body, with particular focus on the floor panel, which is one of the sources of road noise.
For example, numerous beads are formed on the floor panel or the thickness of the panel is increased to raise its surface rigidity and thereby shift its natural frequency to a high frequency band that is higher than 300 Hz. This means that the floor panel is made to not vibrate near 160 Hz, due to suspension vibration, or at the frequency band of tire cavity resonance, and thus road noise is reduced. With these approaches, however, high frequency vibrations subsequently become a problem that requires measures such as attaching sound absorbing material to the floor panel in order to absorb the high frequency noise.
However, the use of large amounts of sound absorbing material increases material costs and makes the automobile body heavier.
In response to these problems, we noticed that sound radiated from a vibrating panel changes significantly depending on the vibration mode, and thus we propose that the floor panel shape and the boundary conditions, for example, are established such that, in specific frequency bands where road noise is a concern, a vibration mode with a low acoustic radiation efficiency is excited. This proposal is described in JP-H09-202269A.
More specifically, the number of antinodes of the stationary waves excited lengthwise and widthwise in a substantially square panel are given as n and m, respectively, and when the vibration mode is nxc3x97m=even number, then sounds radiated from opposite phase, adjacent sections in the panel cancel each other out and are reduced. As shown in FIG. 5B, the acoustic radiation efficiency is lowest particularly when the vibration mode is the 2xc3x972 mode.
Accordingly, in the reduced radiation sound structure of the body panel disclosed in the above application, a substantially square region (vibration mode adjustment region) is set at both the left and right sides of the floor tunnel of the body floor panel, and the distribution of surface rigidity in the panel is adjusted so that the vibration mode of the regions is the 2xc3x972 mode. Consequently, even if vibrations of a specific frequency band caused by tire cavity resonance or suspension vibration, for example, are input and resonate the floor panel, road noise resulting from this is sufficiently suppressed and the degree of silence inside the automobile can be increased.
However, as mentioned previously, the frequency bands that result in road noise are very nearly fixed, and establishing a region in which 2xc3x972 mode vibration is excited with respect to vibrations input in these frequency bands requires that a flat surface with a wide area is secured in the floor panel.
In general, however, a floor tunnel portion extending in the length direction of the body is formed in the body floor panel. Moreover, side frames, side sills, and reinforcing members such as cross members are joined to the body floor panel. These not only ensure automobile body rigidity and increase steering stability but are also critical from the standpoint of increasing the automobile""s ability to protect passengers during impact. Consequently, it is not possible to make large changes to their dimensions, shape, and layout. Therefore, adopting the above floor panel structure for exciting 2xc3x972 mode vibration for the body of the automobile is difficult to achieve because it is difficult in practice to secure a wide flat surface.
The present invention has been arrived at in light of these issues, and it is an object thereof to utilize the layout of the floor tunnel portion and side frames, for example, in the floor panel of an automobile and simultaneously set vibration mode adjusted areas within the floor panel to both ensure body rigidity and safety and reduce road noise by adjusting the vibration mode.
To achieve the above objects, in the present invention, an area of a floor panel partitioned by the floor tunnel portion and the various reinforcing members, and which is oblong in the length direction of the automobile body, is made so that a 2xc3x971 mode vibration resulting in two antinodes in the length direction of the automobile and one antinode in the width direction of the automobile is generated, and is adjusted so that the natural frequency of the 2xc3x971 mode is effective in reducing road noise due to tire resonance.
That is, the present invention is for an automobile floor structure in which the automobile floor is partitioned into a plurality of areas by a floor tunnel portion extending in the lengthwise direction of the automobile along the central portion in the automobile width, left and right side sills extending in the lengthwise direction of the automobile body along both side portions of the automobile width, side frames extending in the lengthwise direction of the automobile body between the floor tunnel portion and the left and right side sills, and cross members extending in the automobile width direction,
wherein the floor panel of at least one area of the plurality of areas is bound at its perimeter by the one of the side frames, two cross members and the floor tunnel portion or one of the side sills, and has a floor panel structure in which the vibration mode is adjusted so that a 2xc3x971 mode vibration resulting in two antinodes in the lengthwise direction of the automobile body and one antinode in the automobile width direction is generated and the natural frequency of the 2xc3x971 mode is 240 to 260 Hz.
According to the invention, vibration of a 2xc3x971 mode occurs in the vibration mode adjusted floor panel when vibrations of between 240 and 260 Hz are input to the floor panel from the outside. This means that in the floor panel, two sections that are adjacent in the length direction of the automobile vibrate with opposite phase but with the same amplitude, so that there is a considerable drop in the acoustic radiation efficiency and thus road noise between 240 and 260 Hz can be significantly reduced.
The perimeter of the vibration mode adjusted floor panel is bound by the floor tunnel portion and strengthening members (area partitioning members) such as the side frames, and thus it easily forms independent vibration systems and is beneficial in exciting the intended 2xc3x971 vibration mode.
The vibration mode adjusted floor panel is also effectively reinforced by the floor tunnel portion extending in the lengthwise direction of the automobile, the side frames, the side sills, and the cross members that intersect with these and extend in the width direction of the automobile. Thus, the automobile body rigidity and its ability to protect passengers can be adequately ensured.
This means that with the present invention, a floor panel structure is formed that utilizes the automobile body reinforcement structure for increasing the automobile""s body rigidity and ability to protect passengers and that employs areas that are partitioned by these reinforcing members (strength members) to generate a 2xc3x971 mode vibration and thereby significantly reduce road noise at 240 to 260 Hz due to tire cavity resonance.
The vibration mode adjusted floor panel can be adjusted to partially increase its rigidity so that stationary wave vibrations in the 2xc3x971 mode are generated at 240 to 260 Hz.
That is, even if a flat panel shape is adopted for the entire floor panel of an area, if the plate thickness is increased to raise the area""s rigidity, then its natural frequency can be adjusted to 240 to 260 Hz. However, this results in a heavier automobile body and a considerable increased in the automobile body weight, particularly if the entire panel constituting the floor is press-shaped from a single plate material.
Accordingly, an approach of partially increasing the rigidity of the vibration mode adjusted floor panel is adopted so that the thickness of the entire plate does not have to be increased. As a result, a reduction in road noise can be achieved without a large increase in the body weight. Moreover, because the approach is that of partially increasing the rigidity of the floor panel, the natural frequency can be easily adjusted to a target value during the design stage of the floor panel. Examples of methods for partially increasing the rigidity include partially forming concave or convex portions in the floor panel, partially increasing the plate thickness, and partially joining other members to the floor panel.
The vibration mode adjusted floor panel can be provided with two rigidity adjustment portions with increased surface rigidity and which are lined up in the length direction of the automobile body so that vibrations in the 2xc3x971 mode occur. Also, the floor can be formed so that the periphery of the rigidity adjustment portions is flat.
Consequently, the center of the two rigidity adjustment portions, which have an increased surface rigidity, becomes an antinode and 2xc3x971 mode vibration is generated in the floor panel. Also, the periphery of the rigidity adjustment portions is formed flat and with low rigidity, and the flat portion flexibly deforms in a vertical direction. Thus, coupled vibration is prevented between the floor panel of the area and surrounding area partitioning members or the floor panels of other areas. Also, this configuration is advantageous in generating vibrations in the 2xc3x971 mode. The section between the two rigidity adjustment portions is also formed flat with low rigidity, so that this intermediate flat portion becomes a node where the site forward of it and site to the rear of it easily vibrate with opposite phase. This means that vibrations in the 2xc3x971 mode are more easily produced.
In this case, it is for example possible for the rigidity adjustment portions and the flat portion around them to have the same plate thickness, and also devise each entire rigidity adjustment portion as a concave surface that recesses downward or a convex surface that protrudes upward. It is also possible to provide a bead at the concave or convex surface to further adjust the direction in which the rigidity is increased.
The vibration mode adjusted floor panel can also be provided with a substantially rectangular panel portion that generates a stationary wave vibration of the 2xc3x971 mode at 240 to 260 Hz.
Vibrations in the 2xc3x971 mode occur easily when the floor panel is rectangular. Accordingly, the floor panel is provided with a substantially rectangular panel portion (region) that vibrates in the 2xc3x971 mode when vibration is given from the outside, so that the natural frequency of the panel portion in the 2xc3x971 mode is 240 to 260 Hz.
When the vibration mode adjusted floor panel is non-rectangular in shape, rigidity adjustment portions that have a higher surface rigidity than other sections are provided at the periphery of the non-rectangular floor panel so that a substantially rectangular panel portion that generates 2xc3x971 mode standing wave vibrations at 240 to 260 Hz can be formed in the non-rectangular floor panel.
That is, if the floor panel of the area is non-rectangular in shape, then it is difficult to produce vibration in the 2xc3x971 mode. To remedy this, the area floor panel can conceivably be devised into a shape where vibration in the 2xc3x971 mode easily occurs by altering the shape or positioning of the floor tunnel portion, the side sills, or other area partitioning members. This, however, makes it difficult to effectively reinforce the automobile body and ensure adequate automobile body rigidity and passenger safety.
Accordingly, a rigidity adjustment portion is provided on at least a portion of the floor panel periphery in order to regulate the vibration region of the floor panel. Thus, a substantially rectangular panel portion that generates stationary wave vibrations at 240 to 260 Hz in the 2xc3x971 mode is formed.
It is possible to provide the vibration mode adjusted floor panel with a rigidity adjustment portion that regulates the region in which the floor panel vibrates, so that the ratio of the width edge of the substantially rectangular panel to its length edge is substantially 1:2.
In other words, vibration in the 2xc3x971 mode occurs easily with a rectangular panel having a width to length ratio of substantially 1:2. Accordingly, the floor panel has been provided with a rigidity adjustment portion in order to form a rectangular panel portion (2xc3x971 mode vibration region) with a width to length edge ratio of substantially 1:2.
Adopting this configuration means that whether the floor panel is rectangular or non-rectangular in shape is no longer a concern. Even if the floor panel were rectangular in shape, when the ratio of its oblong width to length sides is not substantially 1:2, it is possible to provide a rigidity adjustment portion at the periphery of the floor panel in order to form a rectangular panel portion in which the width to length ratio is substantially 1:2.
A rigidity adjustment portion can be provided at the periphery of the vibration mode adjusted floor panel in order to suppress coupled vibration between the substantially rectangular panel portion and at least one of the floor tunnel portion, the side sills, the side frames, and the cross members.
That is, the vibration mode adjusted floor panel forms a vibration system separate from the area partitioning members, namely the floor tunnel portion, the side sills, the side frames, and the cross members. It also forms a vibration system separate from the floor panel of other areas. However, these floor panels and area partitioning members easily generate coupled vibration because the elements making up each of the vibration systems are either continuous or joined to one another.
Accordingly, in the present invention, a rigidity adjustment portion that inhibits coupled vibration between the substantially rectangular panel portion and the area partitioning members is provided at the periphery of the vibration mode adjusted floor panel, so that stationary wave vibrations in the 2xc3x971 mode, where the natural frequency is 240 to 260 Hz, are reliably produced in that panel portion.
In this case, the periphery portion of the floor panel (the space outside the 2xc3x971 mode vibration region) can be effectively employed in forming the above rigidity adjustment portion for preventing coupling.
That is, when a substantially rectangular (in particular, having a width to length edge ratio of substantially 1:2) vibration region, such as one that produces 2xc3x971 mode vibration, has been secured in the vibration mode adjusted floor panel, there is often extra space created at the periphery of that floor panel. This space is not only simply extraneous but also disadvantageous for 2xc3x971 mode vibrations. Conversely, from the standpoint of ensuring body strength, for example, it is generally difficult to dispose the area partitioning members in a way that does not result in this extra space. By providing the above rigidity adjustment portion for preventing coupled vibration, this problem is solved, and moreover, effective use of the space can achieved.
The rigidity adjustment portion in this case can be formed by discontinuously changing the area""s surface rigidity at the periphery of the vibration region (the substantially rectangular panel portion). For example, a structural bead (protruding bar having a U-shaped or V-shaped cross section) that extends in the length direction of the body or the width direction of the automobile and is perpendicular to the direction in which the 2xc3x971 mode vibration waves advance (in this case, these are stationary waves, so that two waves of equal wavelength and amplitude advance in opposite directions) can be formed by press-shaping.
In this case, the area becomes easily bent about the structural bead, that is, there is diminished bending rigidity with respect to bending about the structural bead, so that vibrations in the area are not easily transferred between the vibration region side and the outside (area partitioning member side), and thus coupled vibration is avoided.
The floor panel can be formed by press-shaping a single metal plate that has the total width of that between the left and right side sills, including the floor tunnel portion. At this time, the rigidity adjustment portion can be formed on the panel as a structural bead that extends in the width direction of the automobile.
Press-shaping a single metal plate with the total width of the floor panel, including the floor tunnel portion, means that in principle the floor tunnel portion bulges outward, and the material at that time flows in the width direction of the automobile.
Taking this into consideration, the rigidity adjustment portion is given as a structural bead that extends in the width direction of the automobile, so that the plastic flow of the material during press-shaping takes places smoothly. That is, if the rigidity adjustment portion is a structural bead that extends in the length direction of the automobile body, then the bead portion hinders the plastic flow of the material during press-shaping and shape defects tend to occur, however, by making it a structural bead that extends in the width direction of the automobile, the bead portion does not obstruct the press-shaping properties.
When the vibration mode adjusted floor panel is non-rectangular in shape, a plurality of structural beads that extend in the width direction of the automobile can be provided in a line in the length direction of the body with a spacing therebetween, so as to regulate the width of the vibration region (dimensions in the width direction) of that floor panel in order to form a substantially rectangular panel portion in the floor panel. In particular, it is possible to form a rectangular panel portion (2xc3x971 mode vibration portion) that is oblong in the length direction of the body by arranging the beads so that the position of their end on the vibration region side lines up in a straight line in the length direction of the body.
For example, if the floor panels of areas adjacent to the floor tunnel portion are vibration mode adjusted floor panels, then a spacing in the length direction can be provided between the plurality of beads extending in the automobile width direction spanning from the floor panel to the floor tunnel, and the end portions of the beads can be positioned on the border line of the 2xc3x971 mode vibration region.
Also, the present invention is characterized in that the automobile floor is partitioned into a plurality of areas by a floor tunnel portion extending in the lengthwise direction of the automobile body along a central portion in the automobile width, left and right side sills extending in the lengthwise direction of the automobile body along both side portions of the automobile width, side frames extending between the floor tunnel portion and the left and right side sills in the lengthwise direction of the automobile body, and cross members extending in the automobile width direction, and
a floor panel of at least one area of the plurality of areas is bound at its perimeter by one of the left and right side sills, two cross members and the floor tunnel portion or one of the left and right side frames, and has a floor panel structure in which a vibration mode is adjusted such that a 2xc3x971 mode vibration resulting in two antinodes in the lengthwise direction of the automobile body and one antinode in the automobile width direction is generated and a natural frequency of the 2xc3x971 mode is substantially matched to a cavity resonance frequency of the automobile tires.
Consequently, road noise resulting from tire cavity resonance can be significantly reduced through 2xc3x971 mode vibration by utilizing the floor reinforcement structure of the automobile body for increasing the body rigidity of the automobile and the automobile""s ability to protect passengers while employing the areas that are partitioned by these reinforcing members (strengthening members).
The present invention is also characterized in that the automobile floor is partitioned into a plurality of areas by a floor tunnel portion extending in the lengthwise direction of the automobile along a central portion in the automobile width, left and right side sills extending in the lengthwise direction of the automobile body along both side portions of the automobile width, side frames extending between the floor tunnel portion and the left and right side sills in the lengthwise direction of the automobile body, and cross members extending in the automobile width direction, and
a floor panel of at least one area of the plurality of areas is bound at its perimeter by one of the left and right side sills, two cross members and the floor tunnel portion or one of the left and right side frames, and has a floor panel structure in which a vibration mode is adjusted such that a 2xc3x971 mode vibration resulting in two antinodes in the lengthwise direction of the automobile body and one antinode in the automobile width direction is generated and a natural frequency of the 2xc3x971 mode is 200 to 300 Hz.
In other words, the tire cavity resonance frequency of an automobile is generally within a range of 200 to 300 Hz, although this differs depending on the type of tire that has been equipped to the automobile (for example, tire width, tire diameter, compression, air pressure), the speed of the automobile, and the atmosphere temperature. Therefore, in the present invention, a floor panel structure in which the vibration mode has been adjusted so that the natural frequency of the 2xc3x971 mode is 200 to 300 Hz has been adopted. Thus, a reduction in road noise due to tire cavity resonance can be achieved.
The vibration mode adjusted floor panel can be set between the floor tunnel portion and the side frames underneath the automobile front seats.
With this configuration, the vibration mode adjusted floor panels are disposed underneath the front seats, so that road noise emanating from below the front seats to the passengers sitting therein is effectively reduced. Also, the floor panels are hidden below the front seats, so that 2xc3x971 mode vibration is prevented from being transferred to the feet of passengers sitting in the front seats. Conversely, the feet of passengers are not allowed to interfere with the 2xc3x971 mode vibration in the floor panel. Thus, this configuration is advantageous in reducing radiated noise.
The vibration mode adjusted floor panel can also be formed on both sides of the side frames.
Thus, it is possible to achieve a reduction in radiated noise by effectively utilizing the areas, which are oblong in the length direction of the body, on either side of the side frames.
It is possible to adjust the rigidity of the floor panel of areas of the plurality of areas other than those in which the floor panel has been given a vibration mode adjusted floor panel structure, so that its natural frequency is higher than 300 Hz.
That is, although all of the plurality of areas can be given a vibration mode adjusted floor panel structure, there may also be areas in which it is difficult to generate effective 2xc3x971 mode vibrations due to the relationship of the layout, for example, of the reinforcing members. Accordingly, in these areas, the rigidity of the floor panel is adjusted so that the panel has a natural frequency that is higher than 300 Hz, and therefore the panels avoid resonating with respect to external vibration equal to or less than 300 Hz, and a reduction in radiated sound is achieved.
In an automobile that has been provided with the above floor structure, making the front suspension a double wishbone suspension is beneficial in increasing silence within the interior of the automobile.
As mentioned above, the vibration mode adjusted floor panel is highly effective in reducing radiated sound when vibrations between 200 and 300 Hz are input, however, in an automobile the peak of road noise due to suspension resonance appears around 160 Hz. This road noise is particularly conspicuous with a Macpherson strut suspension (hereinafter, referred to just as strut suspension). This is because with this type of suspension, the bottom end of the damper, the top end of which is joined to the automobile body, is rigidly joined to the knuckle/spindle, so that while the automobile is in motion, front-to-rear and side-to-side vibrations are easily transferred to the automobile body from the knuckle/spindle via the damper.
In contrast, in the case of a double wishbone suspension, an upper arm and a lower arm are joined to the upper and lower ends of the knuckle/spindle by ball joints, and the bottom end of the damper is joined to the upper or lower arm by a ball joint. Therefore, front-to-rear and side-to-side vibrations that are delivered to the damper from the knuckle/spindle via the upper or lower arm are absorbed by vibration of the damper about the point where its upper end is attached to the automobile body, and thus are not easily transferred to the automobile body. Thus, road noise near 200 to 300 Hz is suppressed by vibrations in the 2xc3x971 mode of the vibration mode adjusted floor panel, and road noise near 160 Hz is also diminished. Thus, this configuration is advantageous in increasing the silence within the automobile interior.
Furthermore, the present invention is characterized in that the automobile floor is partitioned into a plurality of areas by a floor tunnel portion extending in the lengthwise direction of the automobile body along a central portion in the automobile width, left and right side sills extending in the lengthwise direction of the automobile body along both side portions of the automobile width, side frames extending between the floor tunnel portion and the left and right side sills in the lengthwise direction of the automobile body, and a plurality of cross members extending in the automobile width direction,
a floor panel of at least one area of the plurality of areas is bound at its left and right by one of the side frames and either the floor tunnel portion or one of the side sills, formed in a substantially rectangular shape that is bound at its front and back by two cross members and oblong in the lengthwise direction of the automobile body, and has long sides that are formed more than twice as long as its short sides, and
the floor panel is provided with a pair of curved surface portions, which are formed in an elliptical shape with a perimeter that is oblong in the lengthwise direction of the automobile body and protrude upwards or downwards, and are lined up in the lengthwise direction of the automobile body with coinciding long axes, and the floor panel has a floor panel structure in which a vibration mode is adjusted such that a 2xc3x971 mode vibration resulting in two antinodes in the lengthwise direction of the automobile body and one antinode in the automobile width direction is generated and a natural frequency of the 2xc3x971 mode is substantially matched to a tire cavity resonance frequency of the automobile.
In the present invention, when a vibration induced by the tire cavity resonance is input to the floor panel, a 2xc3x971 mode vibration, in which the two elliptical curved surface portions that are adjacent to one another in the lengthwise direction of the automobile body vibrate out of phase and at the same frequency, is generated in the floor panel, and thus the acoustic radiation efficiency is severely diminished, so that road noise due to tire cavity resonance can be significantly diminished.
Furthermore, in a case where the floor panel is rectangular in shape and enclosed by the above strengthening members, an ideal rectangle for generating the 2xc3x971 mode vibration is a 2xc3x971 rectangle in which the long sides are twice the length of the short sides. However, depending on the automobile, the layout of the floor tunnel portion, the side sills, the side frames, and the cross members may not allow a floor panel that is surrounded by these strengthening members to become a 2xc3x971 rectangle, and for example, the floor panel may be closer in shape to a 3xc3x971 rectangle, in which case it is difficult to generate a 2xc3x971 mode vibration.
To explain this in greater detail, even if the floor panel is not a 2xc3x971 rectangle, as long as it is close in shape to a 2xc3x971 rectangle, it is possible to provide a floor panel that is substantially a 2xc3x971 rectangle by providing beads and other reinforcing members at its periphery portion. However, if the floor panel is close in shape to a 3xc3x971 rectangle, then the region that remains after reinforcing beads extending in the width direction of the automobile are provided and the floor panel has been partitioned to form a 2xc3x971 rectangular region, has its own characteristic vibration, or coupled vibration is generated between the region that remains and the 2xc3x971 rectangular region, which is unfavorable with regard to reducing the anticipated road noise through 2xc3x971 mode vibration.
Also, as was mentioned above, if the floor panel is a long and thin rectangle, then the bending rigidity in the lengthwise direction of the automobile body (the bending rigidity when the panel bends about an axis in the width direction of the automobile body) is lower than the bending rigidity in the width direction of the automobile body (the bending rigidity when the panel bends about an axis in the lengthwise direction of the automobile body).
Accordingly, in the present invention, the pair of curved surface portions that line up in the lengthwise direction of the automobile body are formed in a substantially rectangular floor panel portion with long sides twice as long as its short sides, have a planar shape that is elliptical and oblong in the lengthwise direction of the automobile body, and increase the bending rigidity in the lengthwise direction of the automobile.
Thus, a 2xc3x971 mode vibration in which the pair of elliptical curved surface portions vibrate up and down in opposite phase to one another is generated in the floor panel, and the curved surface portions are ellipses that are oblong in the lengthwise direction of the automobile body. This is advantageous for substantially matching the natural frequency of the 2xc3x971 mode to the tire cavity resonance frequency.
That is, it is not necessary to provide elliptical curved surface portions if the object is only to achieve 2xc3x971 mode vibration, and for example, the curved surface portions could conceivably be given a substantially rectangular perimeter. However, in order to reduce road noise due to cavity resonance, the surface rigidity of the floor panel must be effectively increased to substantially match its natural frequency in the 2xc3x971 mode with the cavity resonance frequency.
Here, the case of the present invention, where the curved surface portions have an elliptical perimeter instead of a rectangular perimeter, is advantageous in increasing the surface rigidity of the floor panel, because the perimeter of the curved surface portions bypasses the corners of the rectangular floor panel and extends obliquely to link the middle portion of the long sides of the floor panel to the middle portion of the short sides of the floor panel. Moreover, because the curved surface portions according to this invention are ellipses that are oblong in the lengthwise direction of the automobile body instead of having a perfectly circular perimeter, the bending rigidity of the floor panel in the lengthwise direction of the automobile body is effectively increased. Thus, the present invention is advantageous for increasing the natural frequency of the 2xc3x971 mode so that it is substantially matched to the cavity resonance frequency.
Furthermore, the present invention is characterized in that the automobile floor is partitioned into a plurality of areas by a floor tunnel portion extending in the lengthwise direction of the automobile body along a central portion in the automobile width, left and right side sills extending in the lengthwise direction of the automobile body along both side portions of the automobile width, side frames extending between the floor tunnel portion and the left and right side sills in the lengthwise direction of the automobile body, and a plurality of cross members extending in the automobile width direction,
a floor panel of at least one area of the plurality of areas is bound at its left and right by one of the side frames and either the floor tunnel portion or one of the side sills, formed in a substantially rectangular shape that is bound at its front and back by two cross members and oblong in the lengthwise direction of the automobile body, and has long sides that are formed more than twice as long as its short sides, and
the floor panel is provided with a pair of curved surface portions that are formed in an elliptical shape with a perimeter that is oblong in the lengthwise direction of the automobile body and protrude upwards or downwards, and that are lined up in the lengthwise direction of the automobile body with coinciding long axes, and the floor panel has a floor panel structure in which a vibration mode is adjusted such that a 2xc3x971 mode vibration resulting in two antinodes in the lengthwise direction of the automobile body and one antinode in the automobile width direction is generated and a natural frequency of the 2xc3x971 mode is 200 to 300 Hz.
In other words, the tire cavity resonance frequency is generally within a range of 200 to 300 Hz, although this differs depending on the type of tire that has been equipped to the automobile (for example, tire width, tire diameter, compression, air pressure), the speed of the automobile, and the atmosphere temperature. Thus, with the present invention, a pair of elliptical curved surface portions that are oblong in the lengthwise direction of the automobile body are provided in the floor panel and are lined up in the lengthwise direction of the automobile body with coinciding axes, so that a 2xc3x971 mode vibration is generated and the rigidity of the floor panel is adjusted so that the natural frequency of the 2xc3x971 mode is established at 200 to 300 Hz.
Thus, a reduction in road noise due to tire cavity resonance can be effectively achieved.
It is also possible to set the natural frequency of the 2xc3x971 mode to 220 to 240 Hz.
That is, as was mentioned above, although the tire cavity resonance frequency may differ depending on the type of tire that has been equipped to the automobile and the speed of the automobile, for example, in the case of a sports car or the like, the tire cavity resonance frequency is near 230 Hz because of the relationship between, for example, the diameter and the compression of the tires that have been equipped and the automobile speed at which the reduction in road noise is to be achieved. Consequently, the present invention can be adopted for an automobile with a comparatively low tire cavity resonance frequency in order to effectively achieve a reduction in the road noise of that automobile.
It is preferable that each of the pair of elliptical curved surface portions has a large radius of curvature at its central portion and a small radius of curvature at its periphery portion.
Thus, the periphery portion of the elliptical curved surface portions is more upright and has increased rigidity, and this is advantageous for increasing the natural frequency of the 2xc3x971 mode so that it matches the cavity resonance frequency of the tires.
It is preferable that, when viewed from above, the ends of the long axis of the pair of elliptical curved surface portions are in contact with one another or that the ends of the long axes are overlapping.
If the pair of elliptical curved surface portions are shaped so that they are in contact with or overlap with one another, then the curved surface portions can be made larger, and this is advantageous with regard to increasing the rigidity of the floor panel portion so that the natural frequency of the 2xc3x971 mode is substantially matched to the cavity resonance frequency of the tires. Also, even when their shape is such that they overlap, only the end portions of the long axis of the pair of elliptical curved surface portions overlap with one another, and because the shape of the central portion becomes a narrowed gourd-shape if the entirety of both recesses is viewed from above, the narrowed central portion becomes a node and 2xc3x971 mode vibration in which both sides of the node vibrate at an opposite phase can be ensured.
It is preferable that an intermediate bead extending in the lengthwise direction of the automobile body and narrower in width than the short axis of the ellipse is formed in the floor panel, so as to link the end portions of the long axis of the pair of elliptical curved surface portions to one another.
That is, as mentioned above, because the floor panel has a low bending rigidity in the lengthwise direction of the automobile body, its central portion (the site between the adjacent elliptical curved surface portions) becomes severely distorted when vibrated, and that distortion can adversely affect the 2xc3x971 mode vibration.
If an intermediate bead is provided, then, because the rigidity of the central portion of the floor panel is increased, and particularly because the intermediate bead extends in the lengthwise direction of the automobile body, the intermediate bead is effective in increasing the bending rigidity of the floor panel in the lengthwise direction of the automobile body and is advantageous for obtaining 2xc3x971 mode vibration with little distortion, and moreover, it is advantageous for substantially matching the natural frequency of the 2xc3x971 mode vibration to the tire cavity resonance frequency. Also, the width of the intermediate bead is shorter than the short axis of the ellipse, so that both recessed portions and the intermediate bead together produce a gourd-shaped outline in which the intermediate bead is the site that is narrowed, and this is advantageous for 2xc3x971 mode vibration in which the site of the narrowed intermediate bead serves a node.
It is preferable that end portion beads extending in the lengthwise direction of the automobile body are formed in the floor panel at the edge of each elliptical curved surface portion in the direction of its long axis on the side opposite the intermediate bead.
Thus, because of the intermediate bead and the end portion beads, a balance can be achieved in the rigidity of the front and rear end portions of the elliptical curved surface portions, and this is advantageous for orderly vibrating the elliptical curved surface portions up and down without distortion in order to obtain vibration in the 2xc3x971 mode.
It is further preferable that lateral portion beads extending in the lengthwise direction of the automobile body are formed in the floor panel at the side edge of each elliptical curved surface portion.
Thus, the lateral portion beads can be employed to balance the rigidity of both sides of the elliptical curved surface portions, and this is advantageous for vibrating the elliptical curved surface portions vertically in an orderly fashion without distortion in order to obtain vibration in the 2xc3x971 mode.
If the floor panel is bound on its left and right by a side frame and a side sill, the lateral portion beads can be disposed not of the side of the side sills but on the side of the side frames, which have a low degree of binding with respect to the floor panel, so as to balance the rigidity of both sides of the elliptical curved surface portions. Thus, this is advantageous for vibrating each elliptical curved surface portion vertically in an orderly fashion without distortion in order to obtain vibration in the 2xc3x971 mode.
It is preferable that if the floor panel is the above long and thin rectangle, then a rigidity adjustment means, such as the elliptical curved surface portions, the intermediate bead, the end portion beads, and the lateral portion beads, that functions to increase the bending rigidity in the lengthwise direction of the automobile body more so than to increase the bending rigidity in the width direction of the automobile body is provided.
As mentioned above, the floor panel is substantially rectangular in shape and oblong in the lengthwise direction of the automobile body, so that its bending rigidity in the lengthwise direction of the automobile body is lower than its bending rigidity in the width direction of the automobile body. Therefore, by providing a rigidity adjustment means, the rigidity at the front, rear, left, and right of the floor panel can be balanced while its overall rigidity can be increased. This is advantageous for substantially matching the natural frequency of the 2xc3x971 mode to the tire cavity resonance frequency.
In a two-door type automobile, or in a four-door type hinged double door automobile in which the rear doors are more narrow than the front doors, there is a wide space between the cross member at the front end of the floor and the cross member behind it, and a floor panel that is bound by these two front and rear cross members is easily provided substantially rectangular in shape with long sides in the lengthwise direction of the automobile body twice the length of its short sides.
Thus, adjusting the vibration mode of the floor panel of this type of automobile using the pair of elliptical curved surface portions proves advantageous in achieving a reduction in road noise caused by tire cavity resonance.
The present invention is also characterized by a method of designing an automobile floor panel, wherein the automobile floor is partitioned into a plurality of areas by a floor tunnel portion extending in the lengthwise direction of the automobile body along a central portion in the automobile width, left and right side sills extending in the lengthwise direction of the automobile body along both side portions of the automobile width, side frames extending between the floor tunnel portion and the left and right side sills in the lengthwise direction of the automobile body, and a plurality of cross members extending in the automobile width direction, and
a floor panel of at least one area of the plurality of areas is bound at its left and right by one of the side frames and either the floor tunnel portion or one of the side sills, formed in a substantially rectangular shape that is bound at its front and back by two cross members and oblong in the lengthwise direction of the automobile body, and has long sides that are formed more than twice as long as its short sides, the method of designing an automobile floor panel including:
a step of designing a basic floor panel structure in which a 2xc3x971 mode vibration resulting in two antinodes in the lengthwise direction of the automobile body and one antinode in the automobile width direction is generated by disposing, in the floor panel, a pair of curved surface portions that are formed in an elliptical shape with a perimeter that is oblong in the lengthwise direction of the automobile body and protrude upwards or downwards, and that are lined up in the lengthwise direction of the automobile body with coinciding long axes, and
a step of increasing a natural frequency of the 2xc3x971 mode by providing a groove-shaped structural bead that extends in the lengthwise direction of the automobile body and is narrower in width than the short axis so as to link the end portions of the long axis of the pair of elliptical curved surface portions to one another, and of tuning the natural frequency to substantially match the tire cavity resonance frequency of the automobile by adjusting the depth of the groove-shaped structural beads.
As mentioned above, forming a pair of elliptical curved surface portions in a flat, rectangular floor panel is effective in generating 2xc3x971 mode vibration. However, in order to reduce road noise due to tire cavity resonance, the rigidity of the floor panel must be increased so that the natural frequency of the 2xc3x971 mode is substantially matched to the tire cavity resonance frequency. With regard to this, the inventors found that the rigidity of the floor panel can be easily increased by providing groove-shaped structural beads linking the two elliptical curved surface portions in the floor panel. The inventors also found that the rigidity of the panel is easily altered by changing the depth of the groove of the structural beads.
Accordingly, in the present invention, the natural frequency of the 2xc3x971 mode is increased by providing groove-shaped structural beads, and the natural frequency is tuned so that it is substantially matched to the tire cavity resonance frequency of the automobile by adjusting the depth of the groove-shaped structural beads.
Consequently, with this method of designing a floor panel, the natural frequency in the 2xc3x971 mode of the floor panel can be easily matched to the target tire cavity resonance frequency by adjusting the depth of the groove-shaped structural beads. Moreover, the same basic floor panel shape can be adopted even when the tire cavity resonance frequency differs because of the automobile speed and the tire type, and thus the floor panel can be easily designed.